Schistosomiasis is a chronic, debilitating parasitic disease affecting hundreds of millions of people worldwide. The parasite is an extracellular blood fluke that can survive for many years without replicating in its human host. Since the worms must interact closely with their host, performing functions such as nutrient uptake and attachment normally performed by integral surface membrane proteins, the parasites must have developed mechanisms for avoiding immune attack directed at these proteins. Our long-range goal is to identify these mechanisms and attempt to subvert their effectiveness by immunization with recombinant surface membrane protein epitopes. As a first step, we have recently cloned a full-length cDNA encoding the schistosome homologue of EGF receptor (SER) and partial cDNA clones encoding two distinct schistosome homologues of the mammalian multi-drug resistance factor, P-glycoprotein (SMDR1), SMDR2). While studying SER expression, we also found that schistosomes use alternate splicing pathways to generate variant SER transcripts encoding carboxy- terminally truncated forms of SER that should be both secreted and membrane-anchored. These variant forms may modulate the immune response against SER or bind, without consequence, antibodies that might otherwise be damaging to the parasite. The specific aim of this proposal is to characterize extensively the normal expression and immunogenicity of the different SER and SMDR proteins. Expression of SER and SMDR will be studied in terms of transcription, timing during the life cycle and localization of their transcripts and protein products. Full-length, native products will be expressed within mammalian cells and the antibody response against these proteins compared to that within infected mice. Anti-sera and recombinant antibodies against peptide epitopes on these proteins will be generated and tested for their ability to recognize extracellular epitopes on the native membrane proteins and to inhibit the function of these proteins. If the results are promising, in vivo testing of their vaccine potential will be carried out. These studies should provide new insight into the biology of schistosomes and the molecular evolution of two important membrane protein families. In addition, they should provide the basis for additional studies concerning the potential of using serum-exposed schistosome surface membrane proteins as vaccine targets.